One form of antenna widely used for communication and radar purposes is a helical (or helix) antenna. A helical antenna is an antenna that emits or responds to electromagnetic (EM) fields in a circular polarization. Maximum radiation or response is wanted along the axis of the helix, about which the helical coil is disposed. A set of helical antennas may be mounted together to form an array, or phased array, antenna. In customary applications, the spacing in an array is larger than half of a wavelength.
A helical antenna comprises an electrically conductive helical coil that can transmit or receive, or both, EM signals. The antenna properties of the helical coil are a function of several of its physical characteristics, including axial length, turn spacing and diameter (or radius) of the coil. The helical antenna extends orthogonally from a ground plane, at its base (or first) end, to its distal (or second) end. Typical helical antennas either have a uniform radius or they are axially tapered from the antenna's base to its distal end.
The helical coil includes, at its base, a feed line that connects the antenna, through the ground plane, to the receiver, transmitter, or transceiver—depending on the type of antenna. The feed line transfers radio-frequency (RF) energy from a transmitter to an antenna, and/or from an antenna to a receiver, but, if operating properly, the feed line typically does not radiate or intercept energy. In a typical helical antenna arrangement, the feed line is offset from the axis of the helical coil and typically coupled through the ground plane to an amplifier or filter.
Generally, there are three types of commonly used antenna feed lines, also called RF transmission mediums: coaxial line, waveguide and strip line/micro-strip line. These are typically used to transmit or receive RF signals to and from the helical coil. A coax cable is a shielded copper-core channel that carries the signal, surrounded by a concentric second channel cable that serves as ground and is covered by an outer sheathing. A waveguide is a hollow, metallic tube or pipe with a circular or rectangular cross section. The diameter of the waveguide is comparable to the wavelength of the EM field, typically. The EM field travels along the inside of the waveguide. The metal structure prevents EM fields inside the waveguide from escaping, and also prevents external EM fields from penetrating to the interior. Waveguides are used at microwave frequencies, that is, at 1 GHz and above. Strip line or micro-strip lines are planar transmission mediums used in, among other things, RF applications. Strip lines or micro-strip lines may be integral with, mounted on or etched into the ground plane.
A helical antenna is an “axial mode” antenna, meaning it preferably radiates or receives energy primarily along its axis. From an RF perspective, the helical antenna has two primary characteristics that are of importance. The first is amplitude, which is a measure of the magnitude of the RF signal. The amplitude should be at its maximum along the axis of the helical coil. For the most part, amplitude is independent of the rotation of the coil about the axis. The second characteristic is phase, which reflects the frequency characteristics of the signal. Unlike amplitude, the phase of the helical antenna is directly related to the rotational orientation of the helical coil about the axis. For example, a quarter turn of the coil effects a 90° change in phase, a half turn effects a 180° change in phase, and so forth. Thus, the rotational orientation of a helical antenna, particularly within an array of antennas, is important.
In order to achieve the desired radiation pattern for such a helical antenna, whether in an array or alone, a helical antenna may require rotation about its axis. Consequently, once the antenna is located, for example within an array, it may not be freely spun about its axis without adversely impacting antenna performance. For example, rotating such a helical antenna within an array could result in coupling with one or more adjacent helical antennas. Further, with an offset feed line, if the antenna is rotated, then the amplifier/filter typically connected to the feed line would need to be repositioned. This can be particularly time consuming and onerous
Thus, rotation of a helical antenna about its axis could require at least two compensating actions, one is EM related and the other more layout related. First, to eliminate or mitigate undesirable levels of coupling, the physical locations and spacing of the helical antennas within the array may need to be customized. And once placed, any rotation of a helical antenna within the array would likely require modification to the placement of that antenna within the array. Second, since each helical antenna is physically connected to an amplifier/filter module, rotation of the antenna would likely requirement movement or rewiring of the helical antenna to its amplifier/filter module. Thus, current helical antennas having off-set feed lines have limited flexibility.